Variable sensitivity anti-lock brake control

ABSTRACT

The control includes a reference signal with which braked wheel deceleration is compared for sensing incipient wheel lock-up and a circuit responsive to the rate of change of deceleration for changing the reference signal to make the control more or less sensitive to deceleration. The circuit is so arranged that the sensitivity is increased at a low rate of change of deceleration such as occurs during a gradual application of brake pressure on a low coefficient of friction surface.

States Patent [1 1 1111 3,716,273 eyerlein 1451 Feb. 13, 1973 [54]VARIABLE SENSITIVITY ANTI-LOCK 3,554,612 1 1971 l-larned ..303 21 BEBRAKE CONTROL 3,556,614 1/1971 Steigerwald ..303/2l CG [75] lnventor:David G. Beyerlein, Flint, MlCh. Primary yg M Blix [73] Assignee:General Motors Corporation, Assistant Examiner-Stephen G. Kunin Detroit,Mich. Att0meyJean L. Carpenter, Paul Fitzpatrick and 22 Filed: June 1,1970 Warm [21] Appl. No.: 42,304 [57] ABSTRACT The control includes areference signal with which 1.8. CI. CG, A braked whee] deceleration iscompared for ensing in- [Sl] Int. Cl. "1360i cipient whee] lock-up and acircuit responsive to [he Fifild Search rate of change of decelerationfor changing the 340/262 reference signal to make the control more orless sensitive to deceleration. The circuit is so arranged that 1Reielences Clted the sensitivity is increased at a low rate of change ofUNITED STATES PATENTS deceleration such as occurs during a gradualapplication of brake pressure on a low coefficient of friction 3,635,531l/l972 Okarnoto et al. ..303/2l CG surface. 3,545,817 12/1970 Yarber..303/2l P 3,524,685 8/1970 Harned et a1. ..303/20 X 4 Claims, 3 DrawingFigures 1 V r 2 9 VEHICLE EQ E DUAL ADA'PTWE TRACK RELEASE SSE/i l xsgigg ICONDUCTANCE CONTROL Q INTEGRATOR 2d #5 i0 fl 6 4 ./n K F RELEASE[in y A 96 OUTPUT BRAKE SLOW COMPARATOR DRIVER PRESSURE w APPLY 7 \fiMODULATOR INITIAL 63 3 a 90 w CYCLE v VARIABLE SENSITIVITY ANTI-LOCKBRAKE CONTROL This invention relates to an anti-lock brake control andmore particularly, to such a control which is particularly sensitive togradual brake application on low coefficient of friction surfaces.

Anti-lock brake controls frequently are based in their operation on thecomparison of braked wheel deceleration to some predetermined ordeterminable reference value so that when the wheel decelerationachieves a prescribed relationship to the reference signal, the brakepressure is reduced or released to allow recovery of the braked wheeland to avoid locking of that wheel. Usually the reference signal isfixed or varies according to the coefficient of friction of the roadsurface to accommodate varying road conditions. It has been founddesirable, however, to further modify such controls to take into accountthe rate of brake application since in some circumstances, particularlyon low coefficient surfaces and gradual brake application, response ofthe control system can thereby be improved.

It is therefore an object of this invention to provide an anti-lockbrake control having a varying sensitivity to accommodate gradual brakeapplication on low coefficient of friction road surfaces.

It is another object of the invention to provide an anti-lock brakecontrol system having an increased sensitivity when the rate of changeof deceleration of a braked wheel is small.

It is a further object of this invention to provide an anti-lock brakecontrol system having a deceleration reference signal which is variablein response to the rate of change of deceleration of a braked wheel.

The invention is carried out by providing an anti-lock brake controlhaving a varying sensitivity level and a device responsive to the rateof change of wheel deceleration for effecting high sensitivity at lowrates and lower sensitivity for high rates of change of wheeldeceleration.

The invention is further carried out by providing in an anti-lock brakecontrol system a control circuit for comparing a wheel decelerationsignal to a reference signal to effect brake release upon apredetermined relationship of those signals and a circuit responsive tothe rate of change of deceleration of a braked wheel for altering thereference signal to change the sensitivity of the control so that thecontrol is more sensitive to wheel deceleration on low coefficientsurfaces with relatively gradual brake application than on high coef'ficient of friction surfaces or with abrupt brake application.

The above and other advantages will be made more apparent from thefollowing specification taken in conjunction with the accompanyingdrawings wherein like reference numerals refer to like parts andwherein:

FIG. 1 is a graph depicting mu-slip curves for tires on high and lowcoefficient surfaces;

FIG. 2 is a diagrammatic representation of an antilock brake controlsystem according to the invention; and,

FIG. 3 is a schematic representation of a portion of FIG. I,particularly illustrating the invention.

While the subject invention is of general application to variousanti-lock control systems, it is described herein with reference to thesystem described in the patent application of Van Ostrom et al., Ser.No. 806,807, filed Mar. 13, 1969, now abandoned and the correspondingcontinuation Ser. No. 128,484 filed Mar. 26, l97l. Reference may be hadto that application for details not specifically described herein.

FIG. 1 illustrates the well known mu-slip curves which show brakingforce between a tire and a road surface as a function of wheel slip. Thefull line curves A and B depict the mu-slip curves for a highcoefficient of friction surface and a low coefficient surfacerespectively. The salient feature of the high coefficient surface curveA is the peak occurring at about 20 percent slip followed by a negativeslope portion at higher amounts of slip. In contrast, on very slipperyroads as shown in the curve B, there is no significant peak if any tothe curve and after reaching a maximum the curve is essentially flat.Superimposed on the curve of FIG. I are dotted lines C, D, E and F whichrepresent the braking force applied to the wheel for various rates ofapplication of braking pressure supplied by the vehicle operator in theabsence of any intervention by the antilock brake control system. Line Cand E represent fairly abrupt or rapid application of brake pressure fordry surfaces and slippery surfaces, respectively. It is apparent fromthe graph that the line W rapidly diverges from curve A and line Brapidly diverges from the curve B due to the steep slope of the lines Cand E. Line D represents a gradual or slow brake application on a highcoefficient surface and that line also rapidly diverges from curve Abecause of the significant negative slope of curve A following the peak.In each of these three cases, the vertical distance at any given pointrepresents the net decelerating force on the wheel and since theseforces are substantial, the deceleration will be large and will readilybe sensed by an anti-lock control circuit. The rapid divergence of theselines from their respective curves A and B also indicates that the rateof change of deceleration (the second derivative of velocity withrespect to time) is also large. The dotted line F represents the gradualor slow application of brake pressure on a low coefficient surface andit is apparent from the graph that the vertical distance of the line Ffrom the curve B is relatively small indicating a small deceleratingforce on the wheel which can be very difficult for the anti-lock controlto sense. Further, since the line F diverges from the flat portion ofcurve B at a low rate, the rate of change of wheel deceleration will below. Thus the slow brake application condition occurring on a lowcoefficient surface differs from all the other conditions mentioned inthat a small deceleration and a low rate of change of decelerationresult. It is the purpose of this invention to take advantage of thatdifference in the rate of change in deceleration as a control parameterfor altering the sensitivity of the control so that it will respond toall incipient wheel lock-up conditions even those involving low brakedwheel deceleration while retaining a desirable less sensitive responsefor the usual incipient lockup conditions involving high deceleration.

The system for which this invention is utilized follows the wellesTablished principle of sensing incipient wheel lock-up when brakepressure is applied to vehicle brakes, then relieving the brake pressureuntil the wheel accelerates enough to be out of danger of locking andthen reapplying the brake pressure. This cycle is repeated as necessaryto achieve the desired braking action.

Referring to FIG. 2 for a further description of the brake regulatingsystem, a vehicle wheel drive is provided as an information source forthe system. The vehicle wheel drive 10 may be a wheel per se, apropeller shaft driving a plurality of wheels or any other vehiclemember having a velocity or rotation proportional to wheel velocity. Atachometer 12 such as a well known toothed wheel variable reluctanceelectromagnetic transducer is driven by the wheel drive 10 and providesan alternating signal having a frequency proportional to the wheel speedon lines 14 and 16 leading to a frequency-to-voltage converter 18. Thefrequencyto-voltage converter 18 produces on line 20 a linear voltageproportional to frequency and hence proportional to wheel speed, butcontaining some AC ripple. The line 20 is connected to a dualconductance circuit 22 which provides some filtering of the AC ripple inthe wheel speed signal but does not detract from the sensitivity of thefollowing circuit to wheel deceleration. The dual conductance circuit 22is connected by line 24 to an adaptive control circuit 26 which isresponsive to the speed signal and produces an output on line 28 whichroughly simulates vehicle deceleration by sensing wheel decelerationwhen the wheel speed is in substantial synchronism with the vehiclespeed. The line 28 is connected to a track and hold circuit 30 whichmodifies the simulated vehicle deceleration signal on line 28 to providea reference signal on line 32. The track and hold circuit 30 isconnected by line 32 to a release integrator 42. A differentiator 44 isconnected by line 46 to line 24 and by line 48 to the release integrator42. The differentiator 44 senses a wheel speed signal on line 24 anddifferentiates that signal to produce an output on line 48 proportionalto the wheel deceleration.

The release integrator 42 compares the wheel deceleration signal withthe reference signal which corresponds to the Simulated vehicledeceleration signal and integrated the difference to produce on line 50an output proportional to the difference between the simulated vehiclespeed and the wheel speed. This velocity error signal is normally fedunaltered by line 50 to a release comparator 56 which produces an outputsignal on line 58 whenever the velocity error signal exceeds apredetermined maximum. The release comparator includes a hysteresiscircuit which terminates the signal on line 58 when the velocity errorsignal drops below a predetermined minimum value. The line 58 isconnected to an output driver 60 which amplifies the signal on line 58to produce an output on line 62. The line 64 connects the output signalon line 62 back to the track and hold circuit 30 to form an inputthereto.

An initial cycle circuit 66 has an input from the release comparator 56by way ofline 68 and has an output connected by line 70 to the input ofthe release comparator 56. The function of the initial cycle circuit 66is to provide a bias signal to the input of the release comparator 56 torender the release comparator less sensitive to the velocity errorsignal during the first cycle of anti-lock brake operation to insurethat the wheel is approaching an incipient lock-up condition beforeeffecting brake release. When the release comparator 56 produces a firstrelease signal on line 58, it also provides a signal on line 68 whichterminates the output of the initial cycle circuit 66 and succeedingrelease signals on lines 68 and 70 will maintain the initial cyclecircuit 66 off during subsequent cycles of anti-lock brake operation.The output signal on line 62 on the output driver 60 is fed to a brakepressure modulator which is connected by a brake line 82 to a brakemaster cylinder 84 operated by a foot pedal 86. An output brake line 88from the brake pressure modulator 80 is connected to the vehicle brakes90. Normally the brake pressure from the master cylinder 84 is conductedby lines 82 and 88 through the brake pressure modulator 80 to the brakes90 without interference. However, when an output signal is provided online 62 indicating that an incipient skid condition has been sensed bythe control circuit, the brake pressure modulator then reduces the brakepressure in the brake line 88 to release or relieve the braking effort.

To carry out the present invention, a slow apply circuit 92 is added tothe control. This circuit is connected to the output of the adaptivecontrol 26 by a line 94, to the track and hold circuit 30 by line 96 andto the initial cycle circuit 66 by a line 98. The slow apply circuit 92is energized by the initial cycle circuit 66 only during the first cycleof anti-lock brake operation and then only until the first brake releasesince it is only during the first cycle that a slow rate of brakeapplication can occur, the succeeding cycles of braking applicationbeing controlled by the brake pressure modulator which causes brakereapplication at rapid rates of pressure increase. When energized, theslow apply circuit 92 renders the control circuit more sensitive byattenuating the output of the track and hold circuit thereby tending tolessen the value of the reference signal provided to the releaseintegrator 42. By sampling the output of the adaptive control 26, theslow apply circuit 92 senses the rate of change of wheel decelerationand when that rate becomes large, ceases to attenuate the output of thetrack and hold circuit 30. The net effect of the slow apply circuit 92is that the anti-lock control circuit becomes more sensitive to lowwheel decelerations only when the rate of change of deceleration is lowprior to the first brake release.

The operation of the slow apply circuit 92 is more fully described withreference to FIG. 3. The circuitry of FIG. 3 is powered by a line B+connected to a 12- volt vehicle battery, not shown. A resistor 99 and aZener diode 101 are serially connected to ground and their junction isconnected to a line Z+ so that a regulated supply voltage of 8.2 voltsis applied to line 2+.

The purpose of the adaptive control 26 is to provide an output voltagewhich is a useful measure of vehicle deceleration. When the vehiclebrakes are first applied, the wheel deceleration is generallyproportional to the vehicle acceleration until excessive wheel slipoccurs. The adaptive control senses the wheel deceleration and after ashort time delay, provides an output signal on line 28 representing thatwheel deceleration and further representing a reasonable measure ofvehicle deceleration even though after the time delay the wheeldeceleration may differ greatly from the vehicle deceleration. Theadaptive control 26 has an input on line 24 representing wheel velocity,which input is fed to a differentiating capacitor 100. An operationalamplifier comprising transistors 102 and 104 with a feedback circuitcomprising a capacitor 106 in parallel with a resistor 108 completes theadaptive control circuit. The capacitor 100 is connected to the base ofthe transistor 102, the collector of which is connected to the base ofthe transistor 104 and the emitter of which is grounded. The collectorof the transistor 104 is grounded and its emitter, which forms theoutput of the adaptive control circuit, is connected through a loadresistor 110 to the regulated voltage supply Z+. The output of theadaptive control on line 28 then be nearly proportional to the wheeldeceleration and vehicle deceleration except that the output lags behindthe input due to the time delay imposed by the RC constant of thecapacitor 106 and the resistor 108 and the output is limited to a rangeof deceleration from zero to a predetermined maximum, preferably aboutl.l g. The value of the capacitor 106 is selected to be small enough toallow dynamic tracking of vehicle deceleration when the wheel isdecelerating nearly in synchronism with the vehicle and yet large enoughto remember vehicle deceleration when wheel deceleration increasesrapidly. The adaptive control output on line 28 will provide areasonably good measure of wheeldeceleration as long as the wheel andthe vehicle are decelerating nearly synchronously and tends to maintainthat same output for a short period after the wheel beings to deceleraterapidly.

The track and hold circuit 30 is provided to limit excursions of theadaptive control output after a brake release signal is applied to lines62 and 64 and to provide at all times as an output a reference signalrelated to the adaptive control output and representing a simulation ofvehicle deceleration. The track and hold circuit 30 includes a diode 112connected between line 28 and a junction point 114. The junction pointis connected by a capacitor 116 to ground and is further connectedthrough a diode 118 and a resistor 120 to the line Z+. The diode 118 isalso connected to line 64 through a resistor 122. A transistor 124 hasits base connected to the junction point 114, its collector connected toline Z+ and its emitter connected through a resistor 126 to the line 32.In operation, during brake application, the line 64 is at battery or B+potential so that current flows through the diode 118 and 112 to line28. Then the voltage at the junction point 114 and on the capacitor 116is equal to the voltage on line 28 plus the small voltage drop acrossthe diode 112 except for a small time lag required to charge thecapacitor 116 to that voltage. Thus, so long as the brakes are applied,the voltage of the junction 114 will merely track the voltage of line28. The emitter of the transistor 124 will essentially follow thevoltage of the base, the transistor serving only to isolate capacitor116 from the output of line 34 so that the capacitor 116 can hold itscharge. When the output on line 64 approaches ground potential to callfor brake release, the junction of the resistors 120 and 122 will assumea potential less than Z+ by virtue of the voltage dividing action ofthose resistors. Preferably these resistors are selected to provide avoltage of about 4 volts at their junction which corresponds to adeceleration of 0.5 gs. This arrangement then allows the voltage atjunction point 114 to track down according to the voltage at line 28 butwill not allow the voltage to track up beyond four volts due to thebackbiasing of the diode 118. If, however, at the time of brake releasethe voltage at the junction point 114 is higher than four volts, thenthat voltage will be held after brake release by the capacitor 116 ifthe voltage on line 28 does not decrease. Thus the output voltage at theemitter of transistor 124 will be essentially the same as the voltage onthe line 28 except for the limitations imposed by the track and holdcircuit 30 after brake release and except for the small time lagrequired in charging the capacitor 116.

The initial cycle circuit 66 provides a ground potential on line 98until the release comparator 56 produces a brake release signalwhereupon the potential of line -98 approaches B+ potential and the B+potential is maintained throughout the succession of brake release andapply cycles so long as the anti-lock brake operation persists. The line98 is connected through resistor 130 to the base of a transistor 132,which has its emitter connected to 8+ and its collector connectedthrough a load resistor 134 to ground so that the collector will be atground potential when line 98 is at B+ potential and when line 98 is atground potential, the collector will be nearly at B+ potential. Thecollector of transistor 132 is connected to the resistor 136 in the slowapply circuit 92. The resistor 136 in turn is connected to the emitterof the transistor 138 and also is connected through a diode 140 to Z+.Thus during the initial cycle B+ potential is applied to the resistor136 which provides to the emitter of the transistor 138 a voltage equalto 2+ potential plus the voltage drop across the diode 140. The base ofthe transistor 138 is connected through a differentiating capacitor 142to the output line 28 of the adaptive control. It is also connectedthrough a resistor 144 to ground. The collector of the transistor 138 isconnected through a voltage dropping resistor 146 to the base of atransistor which in turn is connected to ground through a base biasresistor 150. The collector of the transistor 148 is connected through acapacitor 151 to the junction point 114 in the track and hold circuit30. A filter capacitor 152 is connected between the base and collectorof transistor 138.

in operation of the slow apply circuit 92, when the transistor 138 isconducting, a voltage approximately equal to 2+ and having a degree oftemperature compensation is generated across resistor 144. The voltagedrop across resistor 144 produces a current through the resistor thatcan be considered a constant reference current. Further, when thetransistor 138 is conducting, the transistor 148 is biased to saturationso that in effect, the capacitor is placed in parallel with thecapacitor 116 of the track and hold circuit. The adaptive control 26produces on line 28 a voltage representative of the deceleration of thevehicle wheel. The input capacitor 142 performs a differentiatingfunction and produces a current roughly proportional. to the rate ofchange of wheel deceleration and this current passes through resistor144. When the current through the capacitor 142 exceeds the referencecurrent through the resistor 144, the transistor 138 is turned offessentially stopping current flow to the base of the transistor andground. Thus when the effective capacitance is increased, the voltage atthe junction 114 follows the voltage of line 28 less closely and duringdeceleration assumes a lower value then if the capacitor 151 were not inthe circuit. This lower voltage consequently decreases the referencesignal on line 32 which renders the release integrator 42 more sensitiveto wheel deceleration. This is so because the release integrator 42subtracts the reference signal on line 32 from the wheel decelerationsignal on line 48 and as the reference signal becomes smaller, theresponse of the release integrator to the wheel deceleration signalbecomes greater. However, when the rate of change of wheel decelerationas sensed by the capacitor 142 exceeds a predetermined value, thecapacitor 151 is effectively removed from the circuit so that thevoltage at the junction point 114 is allowed to more closely track thevoltage on line 28 so that the sensitivity of the release integrator 42is reduced. After the brake release during anti-lock operation, theresistor 136 is grounded due to the operation of the initial cyclecircuit 66 and the slow apply circuit 92 is disabled allowing therelease integrator 42 to maintain its lower sensitivity level.

It will thus be seen that the invention as described provides a way torender an anti-lock brake control circuit very sensitive to wheeldeceleration during the condition of slow or gradual brake applicationon a low coefficient of friction surface and render the control lesssensitive under other conditions where the high sensitivity is notdesirable.

The embodiment of the invention described herein is for the purpose ofillustration and the scope of the invention is intended to be limitedonly by the following claims:

lclaim:

1. in an anti-lock brake control system for a wheeled vehicle havingwheel braking means, a brake regulator means including means for sensingrotation of a braked wheel, and a control responsive to wheel rotationfor effecting brake release to prevent wheel lock-up, the control havinga varying sensitivity level to accommodate road surfaces of differentcoefficients of friction and different rates of brake application, andmeans for selecting the appropriate sensitivity level comprismg:

rate means responsive to rotation of a braked wheel for measuring therate of change of wheel deceleration and means responsive to the rate ofchange of wheel deceleration for changing the sensitivity level of thecontrol,

whereby on low coefficient surfaces and with relatively gradual brakeapplication the control is more sensitive to wheel deceleration than onhigh coefficient surfaces or with abrupt brake application.

2. In an anti-lock system for a wheeled vehicle having braking means,

a brake regulator means including means for sensing rotation of a brakedwheel and a control circuit responsive to wheel rotation for cyclicallyeffecting brake release and brake application to prevent wheel lock-upand maintain optimum braking effort,

the control circuit having a varying sensitivity level on the firstbrake release cycle only to accommodate road surfaces of high or lowcoefficients of friction and rapid or slow rates of brake application,and means for selecting the appropriate sensitivity level comprising,

rate means responsive to rotation of a braked wheel for measuring therate of change of wheel deceleration,

means responsive to the rate of change of wheel deceleration forchanging the sensitivity level of the control circuit to effect a highsensitivity at a low rate of change of wheel deceleration and a lowsensitivity at a high rate of change of wheel deceleration, and

means effective upon the first brake release for disabling thesensitivity selecting means whereby on low coefficient surfaces and withrelatively gradual brake application the control is more sensitive towheel deceleration than on high coefficient surfaces or with abruptbrake application.

3. In an anti-lock brake control system for a wheeled vehicle havingwheel braking means,

a brake regulator means including means for sensing rotation of a brakedwheel and a control circuit responsive to wheel rotation for cyclicallyeffecting brake release and brake application to prevent wheel lock-up,

the control circuit having means for producing a signal as a function ofwheel deceleration, means for producing a reference signal, comparatormeans for comparing the deceleration signal to the reference signal sothat the sensitivity of the comparator means to wheel deceleration isaffected by thevalue of the reference signal,

rate means responsive to rotation of a braked wheel for measuring therate of change of wheel deceleration, and

means responsive to the rate of change of deceleration for attenuatingthe reference signal thereby changing the sensitivity of the control,

whereby on low coefficient surfaces and with relatively gradual brakeapplication the control is more sensitive to wheel deceleration than onhigh coefficient surfaces or with abrupt brake application.

4. In an anti-lock brake control system for a wheeled vehicle havingwheel braking means,

a brake regulator means including means for sensing rotation of a brakedwheel and a control circuit responsive to wheel rotation for cyclicallyeffecting brake release and brake application to prevent wheel lock-up,

the control circuit having means for producing a signal as a function ofwheel deceleration, means for producing a signal as a time delayedfunction of wheel deceleration, a first capacitor responsive to the timedelayed signal for storing a reference signal tending to follow the timedelayed signal comparator means for comparing the deceleration signal tothe reference signal so that the sensitivity of the comparator means towheel deceleration is affected by the value of the reference signal,

alter the value of the stored reference signal and change thesensitivity of the control whereby on low coefficient surfaces and withrelatively gradual brake application, the control is more sensitive towheel deceleration than on high coefficient surfaces or with abruptbrake applica tion.

1. In an anti-lock brake control system for a wheeled vehicle havingwheel braking means, a brake regulator means including means for sensingrotation of a braked wheel, and a control responsive to wheel Rotationfor effecting brake release to prevent wheel lock-up, the control havinga varying sensitivity level to accommodate road surfaces of differentcoefficients of friction and different rates of brake application, andmeans for selecting the appropriate sensitivity level comprising: ratemeans responsive to rotation of a braked wheel for measuring the rate ofchange of wheel deceleration and means responsive to the rate of changeof wheel deceleration for changing the sensitivity level of the control,whereby on low coefficient surfaces and with relatively gradual brakeapplication the control is more sensitive to wheel deceleration than onhigh coefficient surfaces or with abrupt brake application.
 1. In ananti-lock brake control system for a wheeled vehicle having wheelbraking means, a brake regulator means including means for sensingrotation of a braked wheel, and a control responsive to wheel Rotationfor effecting brake release to prevent wheel lock-up, the control havinga varying sensitivity level to accommodate road surfaces of differentcoefficients of friction and different rates of brake application, andmeans for selecting the appropriate sensitivity level comprising: ratemeans responsive to rotation of a braked wheel for measuring the rate ofchange of wheel deceleration and means responsive to the rate of changeof wheel deceleration for changing the sensitivity level of the control,whereby on low coefficient surfaces and with relatively gradual brakeapplication the control is more sensitive to wheel deceleration than onhigh coefficient surfaces or with abrupt brake application.
 2. In ananti-lock system for a wheeled vehicle having braking means, a brakeregulator means including means for sensing rotation of a braked wheeland a control circuit responsive to wheel rotation for cyclicallyeffecting brake release and brake application to prevent wheel lock-upand maintain optimum braking effort, the control circuit having avarying sensitivity level on the first brake release cycle only toaccommodate road surfaces of high or low coefficients of friction andrapid or slow rates of brake application, and means for selecting theappropriate sensitivity level comprising, rate means responsive torotation of a braked wheel for measuring the rate of change of wheeldeceleration, means responsive to the rate of change of wheeldeceleration for changing the sensitivity level of the control circuitto effect a high sensitivity at a low rate of change of wheeldeceleration and a low sensitivity at a high rate of change of wheeldeceleration, and means effective upon the first brake release fordisabling the sensitivity selecting means whereby on low coefficientsurfaces and with relatively gradual brake application the control ismore sensitive to wheel deceleration than on high coefficient surfacesor with abrupt brake application.
 3. In an anti-lock brake controlsystem for a wheeled vehicle having wheel braking means, a brakeregulator means including means for sensing rotation of a braked wheeland a control circuit responsive to wheel rotation for cyclicallyeffecting brake release and brake application to prevent wheel lock-up,the control circuit having means for producing a signal as a function ofwheel deceleration, means for producing a reference signal, comparatormeans for comparing the deceleration signal to the reference signal sothat the sensitivity of the comparator means to wheel deceleration isaffected by the value of the reference signal, rate means responsive torotation of a braked wheel for measuring the rate of change of wheeldeceleration, and means responsive to the rate of change of decelerationfor attenuating the reference signal thereby changing the sensitivity ofthe control, whereby on low coefficient surfaces and with relativelygradual brake application the control is more sensitive to wheeldeceleration than on high coefficient surfaces or with abrupt brakeapplication.